
<html>
<head><title>Calculator</title></head>
<body>

<h1>Calculator</h1>
(Ver 1.0 4/8/2012 by Muhuan Huang)<br />
Type an expression in the following box (e.g., 10.5+20*3/25).
<p>
<form method="GET">
<input type="text" name="expr"><input type="submit" value="Calculate">
</form>
</p>
<ul>
<li>Only numbers and +,-,* and / operators are allowed in the expression.
<li>The evaluation follows the standard operator precedence.
<li>The calculator does not support parentheses.
<li>The calculator handles invalid input "gracefully". It does not output PHP error messages.
</ul>
Here are some(but not limit to) reasonable test cases:
<ol>
  <li> A basic arithmetic operation:  3+4*5=23 </li>
  <li> An expression with floating point or negative sign : -3.2+2*4-1/3 = 4.46666666667, 3+-2.1*2 = -1.2 </li>
  <li> Some typos inside operation (e.g. alphabetic letter): Invalid input expression 2d4+1 </li>
</ol>
<?php
if($_GET["expr"]){
	$valid = 1;
	$str = $_GET["expr"];
	if(!preg_match('/^[0-9\+\-\*\/\. ]+$/', $str)){
		$valid = 0;
	}else{
		if(@eval("\$ans = $str;") === FALSE){	
			$valid = 0;
		}
	}
	echo "<h2>Result</h2>";
	if($valid == 0){
		echo "invalid input expression: ".$str;
	}else{
		echo $str." = ".$ans;
	}
}
?>
</body>
</html>

